The calculation interval is a performance versus accuracy setting. Depending on how much members and visitors your website have, the default settings of the module can cause unneeded processing time and CPU load.
On the other hand we wanted to avoid the need of a cronjob.
Every time the website is loaded by a web browser the module calculates the time to the birthday of your members. This is OK for most websites. If you have a lot of visitors this can cause unneeded CPU load. For this we have create the second option.
One time per day
If you choose calculate one time per day, the module saves the timestamp in the database when it was calculated and after 24 hours he calculates the next time.